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Our approach to the search for life in the universe is highly influenced by our knowledge of life at home on Earth. In Life: How to Make a Cosmic Omelet, The DNA Files looked at the ways in which genetic tools have helped define life as we know it, from bacteria found in a boiling hot spring in Yellowstone National Park to microbes found frozen (and alive!) in the Arctic. Discoveries made on Earth — from microbes that thrive in extreme temperatures to minerals unlike any ever seen before — have fueled our curiosity about life elsewhere in our Solar System and the universe at large.

In 1999, NASA launched the Stardust mission. Stardust is the first NASA mission dedicated to the exploration of a comet (Wild-2, pronounced Vilt-2), and the first mission designed to return extraterrestrial material from outside the orbit of the Moon. Stardust returned to Earth in 2006 after traveling 2.9 billion miles. Using a substance called aerogel, Stardust captured comet samples and stored them for safe return to Earth. This silica-based gel, nicknamed frozen smoke, was inserted into the Aerogel Collector Grid, which looks like a large tennis racket. Researchers analyzing the particles collected by Stardust discovered two kinds of nitrogen-rich organic molecules. According to Scott Sandford, an original co-investigator on the Stardust mission, the samples from Wild-2 are unchanged since the birth of our solar system 4.6 billion years ago. These organic compounds enable researchers to study the origin and evolution of our solar system and potentially, life on Earth.

A little closer to home, scientists are looking to Mars for signs of life. Water is an essential component of life as we know it, and for years, scientists have speculated that Mars once had flowing water on its surface. In 2007, the Spirit mission provided the needed proof. Soil analyzed by the Spirit rover revealed large amounts of silica, which only could have formed if water once flowed on the surface of Mars. Steve Squyres, principal scientist for the Mars Exploration Rover mission, said, "It doesn't mean life was there....But this was a habitable place." The next step is to search these habitable areas for signs of past life, which Squyers believes were, at most, simple microbial organisms.

From a once habitable planet, to our currently comfortable home, to the primordial tale of a comet, researchers continue to search the vast universe for signs of life and clues into "how" we are here.

Original Program Description, 2001

Over the past two years, the National Aeronautics and Space Administration (NASA) has launched a major initiative aimed at unraveling some really big questions:

— Where and how did life begin on Earth?
— Where else might life be found in the universe?

Dubbing their new field "astrobiology," scientists from the disciplines of biology, geology, and chemistry are collaborating on projects they hope will yield answers. Genetic science is one of the main tools they are using.

In this program, John Hockenberry explores scientific research on the origins of life and where else in the universe life is found.

So how do scientists think life began on Earth?

One thing most agree on: at life's beginning, there had to be a cycle that involved some sort of molecules — probably DNA or RNA — that had the capability to encode information, a quality which would make replication possible. There also had to be proteins, some sort of energy source, some sort of container, and some form of metabolism.

That's where the agreement ends. The point of contention: which of these features came first?

To investigate this question, scientists are studying life forms in environments they suppose are not unlike that of Earth before life existed at all. Much of the work to discover life elsewhere in the universe takes place in Earth's most extreme environments - the boiling hot springs of Yellowstone National Park, Devon Island and the deep caves around the world.

In these places, where scientists thought life was impossible, they have found creatures that can endure temperatures above boiling and below freezing. Their discoveries of these "extremophiles" offer hope that explorers may find not little green men, but microbes on other planets such as Mars and on Jupiter's moon, Europa.

To help us learn about life in other parts of the universe, producer Robin White takes us to observe an international expedition to Devon Island in the frozen North Pole. Scientists there are studying microbes around a large impact crater and inside the rocky terrain, in hopes that what they learn here may help them know where to look for life on Mars.

Producer Dan Grossman goes searching for life below the surface of our planet with renowned biologist Penelope Boston. She's famous for finding life forms in the most hostile underground environments like of such as sulphuric and hydrochloric acid. She takes us on a trek through New Mexico's Spider Cave, where she is studying extremophiles that appear to be eating minerals from the cave walls.

But what would it mean if we found microbes on other planets?

We might learn that DNA is a universal code for all life. Or, that some forms of life on Earth came from Mars. Even more intriguing, we might learn there are other systems upon which life is based.

Maybe this type of conclusion wouldn't sink in right away, but it might, eventually, says physicist Paul Davies. Finding out we're not alone could change how we view ourselves and our place in the cosmos as fundamentally as Copernicus did when he recast the sun, not the Earth, as the center of our solar system, he says.